This application claims the priority of Japanese Patent Applications No. 139899/1992 filed Apr. 30, 1992 and No. 198534/1991 filed, Jul. 12, 1991, which are incorporated herein by reference.
A fiber-optic gyroscope obtains an angular velocity of a sensing coil by utilizing the property that the phase difference between a clockwise-propagated light beam and a counterclockwise-propagated light beam is in proportion to the coil angular velocity. Different modes of fiber-optic gyroscope are known owing to the method how to modulate the light beams; phase-modulating method, frequency modulation method and phase shift method, etc.
Since a fiber-optic gyroscope makes the clockwise-propagated light beam and counterclockwise-propagated light beam interfere each other, both light beams must have the same polarization planes at a detector. Difference of the polarization planes will reduce the power of the interference light beam in proportion to cosine of the angle between the different polarization planes. Furthermore, if the polarization planes are vertical each other, they cannot interfere any more.
The polarization planes must be arranged in the same direction at the detector. Since two degenerated light beams having different polarization planes are propagated in a single mode fiber with the same phase constant, the polarization planes are likely to rotate spontaneously.
To suppress the probable rotation of the polarization planes, an improvement of constructing the fiber coil with polarization maintaining fibers and of polarizing the signal light beam before it is divided into two partial light beams may be proposed. Since the polarization maintaining fiber suppresses the rotation of the polarization planes, it can make two partial light beams interfere on the same polarization plane.
Polarization maintaining fibers are not rotationally symmetric, but asymmetric in an angular direction. Asymmetry is obtained by inserting stress applying parts in a diametrical direction or by deforming cores into an elliptic section. Such asymmetry induces a difference of phase constants between two light beams with different polarization planes. Phase velocity will differ according to the polarization. The polarization maintaining fiber has of course birefringence. Thus, it is often called a birefringence fiber. In this description the polarization maintaining fiber will be used as a synonym of the birefringence fiber.
However, the polarization maintaining fiber is much more expensive than the ordinary single-mode fiber. Such a fiber-optic gyroscope which is fully constructed with the polarization maintaining fiber will be highly expensive. Practical significance is poor.
A fiber coil and most of the optical paths shall be preferably fabricated by ordinary single mode fibers. But ordinary single mode fibers have some problems to be solved. Although they are called single mode fibers, it means that a single phase constant has only a single mode. Actually, there are two modes with different polarization planes vertical to each other which have the same phase constant. Two modes with different polarization planes are ideally independent. But polarization planes may rotate owing to external stress or distortion, because the phase constants are the same. The same phase constants degenerate two modes. They cannot forbid the rotation of the polarization planes. The rotation of the polarization planes will mix two modes which would be ideally independent.
However, two modes have different fluctuations of the phase constants in microscopic scale in spite of the macroscopic equivalence of phase constants. Therefore, when two modes have been propagated by the same distance, effective optical path lengths are different. The difference of the effective optical path lengths will induce a fluctuation of the output of the photodetector.
Clockwise propagated light beam and counterclockwise propagated light beam must experience the same path length rigorously in a fiber-optic gyroscope. In microscopic scale, the fluctuation of the polarization would induce a fluctuation of an optical path length. In order to suppress the fluctuation of the optical path length, it shall be effective to fix the polarization plane to a certain direction by launching the light beam through a polarizer before being divided into clockwise propagated light beam and counterclockwise light beam. Such polarization allows only a single mode with a single polarization plane to pass through the single mode fiber. Since single mode beams propagated in a single mode fiber, the optical path lengths are absolutely the same. The situation would be the same as the polarization maintaining fiber mentioned before, if no rotation of polarization occurred. However, a single mode fiber cannot prevent the light beams from rotating polarization planes. Thus, the preparatory polarization by the polarizer is not sufficient to equalize the optical path lengths in any cases. Two light beams with linear polarization planes are propagated in a fiber coil and pass through the same polarizer in a reverse direction. Owing to a probable rotation of the polarization planes, the polarization planes of the beams are not necessarily coincident with the direction of the optical axis of the polarizer. The inclination angle between the optical axis of the polarizer and the polarization plane of the beam is denoted by .PSI.. The amplitude of the beam passing through the polarizer in the reverse direction reduces in proportion to cos .PSI.. The inclination angles are not necessarily the same for the clockwise propagated beam and counterclockwise propagated beam. Furthermore the inclination angle would change owing to a temperature fluctuation. Since the linearly polarized beams often rotate the polarization planes, the output of the photodetector will fluctuate owing to the polarization rotation. Such a fluctuation of the output of the photodetector will hinder an exact measurement of angular velocity.
Therefore, K. Boehm et al. had proposed a single mode fiber-optic gyroscope having a depolarizer in the optical path besides a polarizer. A depolarizer is a device for depolarize arbitrary linearly polarized or elliptically polarized beams. Depolarized state means that polarization planes are distributed with equal probability into all directions. K. Boehm et al.; "Low-Drift Fiber Gyro Using Superluminescent Diode", ELECTRONICS LETTERS, vol. 17, No. 10, p 352 (1981).
FIG. 15 shows the structure of the fiber-optic gyroscope of Boehm. A light source (1) emits a light beam. The beam passes through a lens (21), a beam splitter (22), a polarizer (23) and a lens (24) and enters an end of an optical fiber (25). The optical system converges the light beam on a small fiber core. The beam is linearly polarized by the polarizer (23). Namely, a mode with a single polarization plane is introduced into the fiber. The fiber (25) is coupled to another fiber (27) by a coupler (26). The coupler (26) divides the beam into a clockwise propagated beam and a counterclockwise propagated beam. The clockwise propagated beam once goes out and passes through a lens (28), a depolarizer (29) and a lens (30). The beam converges on an end of the fiber (27) and is propagated in a fiber coil (4) clockwise. Then the clockwise beam passes through a phase modulator (5). The counterclockwise propagated beam is modulated first by the phase modulator (5) and is propagated in the fiber coil (4) counterclockwise. In brief a clockwise propagated beam and a counterclockwise propagated beam are denoted simply by a CW beam and a CCW beam from now on. The CCW beam passes through the depolarizer (29) at last. A depolarizer is an optical device for depolarizing arbitrary linear polarization or elliptical polarization into non-polarization in which polarization planes are distributed in all directions with equivalent probability. The function of a depolarizer is reverse to that of a polarizer. The depolarizer (29) shown in FIG. 15 is called a Lyot depolarizer. FIG. 16 demonstrates a schematic view of a Lyot depolarizer.
A Lyot depolarizer consists of two birefringent crystals coupled each other. Optical axes of two crystals are twisted by 45 degrees. The thicknesses of the crystals are 1:2 in proportion. The thickness of the crystal, is determined so as to keep a difference of optical path lengths between an ordinary ray and an extraordinary ray surpassing the coherent length of the light. FIG. 16 shows isolated crystals for demonstrating optical axes, but in practice, two crystals are glued together without clearance.
The function of a depolarizer will be briefly explained. A linearly polarized beam (a) enters a first birefringent crystal (Q.sub.1) and is divided into two beams (b) and (c) whose polarization planes are in parallel with the optical axes of the birefringent crystal. One is an ordinary beam and the other is an extraordinary beam. The optical path difference L of two beams in the full length the crystal must be longer than the coherent length of the light. Of course the amplitudes of two beams in the crystal (Q.sub.1) are different. Two independent beams enter a second birefringent crystal (Q.sub.2) whose optical axes are inclined at 45 degrees to that the first crystal (Q.sub.1). Both beams (b) and (c) are in halves divided into ordinary beams (d) and (e) and extraordinary beams (f) and (g). The beam (d) and the beam (f) originated from (b) have the same amplitude. The beam (e) and the beam (g) originated from (c) have the same amplitude also. The polarization of the ordinary beams is designated by X-axis. The polarization of the extraordinary beams is designated by Y-axis. The total energy of the light beams with the polarization in parallel with X-axis is equal to a sum of the energy of the beams (d) and (e). Similarly the total energy of the light beams with the polarization in parallel with Y-axis is equal to a sum of the energy of the beams (f) and (g). The energy of the beams with X-polarization is equal to the energy of the beams with Y-polarization, because they are the same sums of halves of the same amounts. In this case, the reason why the total energy of the beams with the same polarization is given by the sum of energy is because the difference of optical paths of the beams with the same polarization is longer than the coherent length the light. The cross terms in the sum of the amplitudes of the different beams vanish in the expression of energy.
The amplitudes of the beams with X- and Y-polarizations are always the same. Thus, the energy of an arbitrary beam is also equal to that of a corresponding beam with the polarization vertical to the former one. Therefore, all beams with any polarization have the same amplitude in common. This state is a depolarized state. Arbitrary linearly-polarized beams are converted to a depolarized state. Thus arbitrary elliptically polarized beams can be also converted into a depolarized state.
Since the second birefringent crystal is twice as thick as the first one, the differences of the optical path lengths of ordinary beams and extraordinary beams regarding four beams (d), (e), (f) and (g) are the same. The differences are in any cases longer than the coherent length. The ratio of the thicknesses shall not necessarily be 1:2. Other ratios except 1:1 are also allowable. But in this case, every difference of the optical path lengths must be longer than the coherent length.
If we wish to use a thinner depolarizer, a light source with a shorter coherent length shall be required.
The fiber-optic gyroscope shown by FIG. 15 is constructed by single mode fibers, a polarizer and a depolarizer. The polarizer and the depolarizer have been made use of for solving the problem of the variation of the output induced by the rotation of the polarization plane. Besides Boehm, similar fiber-optic gyroscopes with a Lyot depolarizer inserted near a fiber coil have been proposed by other persons.
However, the fiber-optic gyroscope shown by FIG. 15 of Boehm was only a device for experiments constructed in a laboratory. It is not a practical device for practical use. Bulk optical parts have been used for a polarizer, a depolarizer and a beam splitter. Such parts are far bigger than fibers. Besides the bulky parts, lenses must be disposed before and behind the parts in order to transform the beams into wide, uniform plane waves. The big discrete parts will make a device too bulky to be used in a car, or another moving object.
A practical fiber-optic gyroscope shall require to reduce the size of polarizer, depolarizer and beam splitter to compact size at least smaller than a fiber coil.
A depolarizer and a polarizer can be made from optical fibers. The fact has been well known. A beam splitter can also be fabricated by optical fibers. Only fabricating all the optical parts by optical fibers shall allow us to make a practical fiber-optic gyroscope for the first time.
A fiber-made beam splitter can be fabricated by melting parts of two fibers, coupling them on sides and expanding the coupled part to enable them to couple evanescently. Such an evanescent coupling divides the light beam into two partial waves. A length of the coupling should be determined to divide the beam rigorously into halves to two branches of an output side.
A depolarizer can also be made from polarization maintaining fibers. FIG. 17 is a schematic view of the depolarizer constructed by two polarization maintaining fibers whose lengths are 1:2 in proportion. Two polarization maintaining fibers are spliced at ends with optical axes twisting together at about 45 degrees. Lengths of the fibers are determined so that the difference of the optical paths of the beams having different polarization planes in parallel with different optical axes is longer than the coherent length off the light. Black dots show the stress applying parts inserted in the fibers in diametric positions. The line connecting two paired dots is ones of the optical principal direction of the fibers. This axis is denoted by X-axis. The other principal axis, Y-axis is vertical to X-axis. The direction of propagation is Z-axis. X- and Y-axes off one fiber are twisted at 45 degrees to X- and Y-axes of the other fiber at the junction. Although FIG. 17 demonstrates dissembled fibers to show the optical axis and the twisting angle, two fibers are spliced at the junction.
A refractive index of the light beam with a polarization plane in parallel with X-axis ( in short, X-polarization )is denoted by n.sub.x. A refractive index of the beam with Y-polarization is denoted by n.sub.y. L.sub.1 and L.sub.2 are lengths of the fibers. The coherent length of the light source is denoted by h. The parameters require the following inequalities in order to make the fiber coupling a depolarizer; EQU .vertline.n.sub.x -n.sub.y .vertline.L.sub.1 &gt;h(i=1,2).vertline.n.sub.x -n.sub.y .vertline. .vertline.L.sub.2 -L.sub.1 .vertline.&gt;h(1)
As a fiber-made polarizer a coil of a polarization maintaining fiber is known. The coil is fabricated by winding a polarization maintaining fiber around a core several turns. Since a polarization maintaining fiber having birefringence is wound, the micro-bending loss differs regarding the directions of the polarization planes. Thus it allows a single beam with a certain direction of polarization to pass through. The other beam with the other direction of polarization vanishes. Thus the coil works as a polarizer. This is called a fiber-type polarizer.
A compact polarizer utilizing thin metal layers has been proposed. FIG. 18 demonstrates such a polarizer consisting of metal films and dielectric films deposited by turns. Metal films have a thicknesses of several nanometers to tens of nanometers. The dielectric films have a thickness of tens of nanometers to hundreds of nanometers. A light beam shall be introduced into the polarizer with a propagating line parallel with the planes of the films. For example, Japanese patent laying open No. 60-97304 (97304/85) has disclosed such a multilayered polarizer. A beam with a polarization plane parallel with the metal films will soon be attenuated. Only a beam with a polarization vertical to the film planes can pass through the device without loss. Thus the multilayered metal dielectric device acts as a polarizer. The metal dielectric multilayered polarizer can be so small that fibers can be spliced directly to the ends of it.
Of course other polarizer can be used. In the case of a discrete, bulky polarizer, two lenses are used to expand a beam to wide plane wave and to converge the wide wave to a core of a fiber as shown in FIG. 19.
The discrete optical parts shown in FIG. 15; a polarizer, a depolarizer and a beam splitter can be replaced by fiber-type devices, or compact devices. However, such replacement is still insufficient to fabricate a practical fiber-optic gyroscope. The problem of fluctuation of the output has not still fully resolved.
The light beam emitted from a light source is a linearly polarized beam. The polarization plane sometimes rotates by some reasons between a light source and a polarizer. It is difficult to harmonize the polarization of a beam with the optical axis of the polarizer at all times. External magnetic force or external tension would induce accidental rotation of polarization. Discrepancy between the polarization and the optical axis reduces the amplitude of the beam passing through the polarizer. In the case of a discrete polarizer as shown in FIG. 15, an initial adjustment of the axes can easily be carried out by maximizing the output of a photodetector by rotating the bulk, discrete polarizer. However, in the case of a fiber-type polarizer or a metal-dielectric multilayered polarizer, such an initial adjustment would be totally impossible, because no light will pass through the polarizer before it is coupled to fibers but no adjustment could be done after it has been coupled to fibers.
Even if such an initial adjustment of harmonizing the beam polarization with the optical axis of the polarizer is carried out, the polarization planes of beams will sometimes rotate by the variation of temperature or distortion of fibers, since an ordinary single mode fiber cannot prevent the polarization from rotating spontaneously. Rotation of the polarization planes will induce an increase or a decrease of the light beam passing through the polarizer. Therefore, the replacement of a discrete, bulk polarizer by a compact polarizer which can propagate a beam after it has been coupled to fibers would cause a new difficult problem of rotation of the polarization between a light source and a polarizer. No initial adjustment can easily be done. A post-adjustment is fully impossible, since the polarizer has tightly coupled to the fibers.
To solve the difficulty, the Inventors had devised a new compact type optic-fiber gyroscope having another depolarizer between a light source and a polarizer. The newly-added depolarizer can be installed either between a light source and a first fiber coupler or between a first fiber coupler and a polarizer. Such a fiber-optic gyroscope had been disclosed by;
1. Japanese patent laying open No. 2-225616(225616/90) PA1 2. Japanese patent laying open No. 2-225617(225617/90) PA1 3. Japanese patent laying open No. 2-225618(225618/90)
Invention 1 employed a depolarizer having two polarization maintaining fibers coupled by an about 45 degree twisting angle. The ratio of lengths of fibers was 1: 2. The length of the shorter fiber was determined so that the difference of effective optical paths between an ordinary beam and an extraordinary beam ( optical path difference by birefringence, in short) is longer than the coherent length of the light source. Such a depolarizer was well-known and equivalent to the depolarizer demonstrated in FIG. 17.
Invention 2 substantially constructed an effective depolarizer near a light source by positioning a polarization maintaining fiber in front of the light source with the optical axis twisted at about 45 degrees to the polarization of the light emitted from the source. Invention 2 dispensed with another polarization maintaining fiber by making use of the twist between the light source and the fiber.
Invention 3 also substantially constructed an effective depolarizer by positioning a birefringent crystal in front of a light source with the optical axis twisted at about 45 degrees to the polarization of the light emitted from the source. Similarly to invention 1, the polarization of the light source and the birefringence of the crystal constituted an effective depolarizer.
The Inventors believe that another depolarizer should be inserted between a light source and a polarizer, when a single mode fiber exists between a light source and a polarizer. Inventions 1, 2 and 3 were based upon the belief. Such a fiber-optic gyroscope was a novel device, because no prior gyroscope contained two depolarizers.
However, invention 2 and invention 3 required a difficult adjustment of optical axes of fibers or birefringent crystals to a polarization plane of a beam emitted from a source. Invention 1 relied upon an independent depolarizer with two polarization maintaining fibers coupled together with optical axes twisted at about 45 degrees. The fiber-type depolarizer is a simple, reliable depolarizer. However, it required many junction operations; a junction of a single mode fiber to a depolarizer, a junction of a polarizer to a depolarizer and junction between two polarization maintaining fibers; at least three junction operations were required.
Such a problem of junction operations accompanies also another depolarizer coupled to an end of a fiber coil besides the depolarizer before the polarizer. Boehm had proposed a depolarizer utilizing two birefringent crystals as shown in FIG. 15. The crystal depolarizer can easily be replaced by a fiber-type depolarizer having two polarization maintaining fibers. The replacement never alleviates the time of junction operations, because a fiber-type depolarizer requires three junctions, i.e. two junctions between an ordinary single mode fiber and a polarization maintaining fiber and one junction between two polarization maintaining fibers. Especially, the junction operation between two polarization maintaining fibers is very difficult, because the optical axes must be twisted at 45 degrees precisely.
However, junction operations can be simplified. Consideration on the function of a depolarizer allows us to simplify the structure of a depolarizer. One purpose of this invention is to provide a fiber-optic gyroscope having simplified depolarizers. Another purpose of this invention is to provide a fiber-optic gyroscope which enjoys low parts cost and low assembly cost by using simplified depolarizers.